Python (data structure and algorithm [3])

Python (data structure and algorithm [3])

Maps keys to multiple values in the dictionary.

One-click multi-value dictionary

d = {'a':[1,2,3], 'b':[4,5]}e = {'a':{1,2,3}, 'b':{4,5}}

You can use the from collections import defaultdict to use the default dictionary class. One feature of the default dictionary class is to automatically initialize the first value. You only need to pay attention to adding elements later.

from collections import defaultdictd = defaultdict(list)d['a'].append(1)d['a'].append(4)...d = defaultdict(set)d['a'].add(5)...

Example:

d = defaultdict(list)for key, value in pairs: d[key].append(value)

Keep the dictionary in order

from collections import OrderedDictd = OrderedDict()d['grok'] = 4d['foo'] = 1d['abr'] = 2for key in d: print(key, d[key])

Output:

grok 4foo 1abr 2

When the dictionary is iterated, it will strictly follow the initial addition order of the elements.

Dictionary-related computing problems

Prices = {'acme ': 45.23, 'aap': 612.78, 'ibm': 205.55, 'hpq': 37.20, 'fb ': 10.75} # Use zip () reverse the dictionary key and value to min_price = min (zip (prices. values (), prices. keys () max_price = max (zip (prices. values (), prices. keys () print (min_price) print (max_price) # To sort data, you only need to use the sorted function price_sorted = sorted (zip (prices. values (), prices. keys () for key, value in price_sorted: print (key, value)

Output:

(10.75, 'FB')(612.78, 'AAPL')10.75 FB37.2 HPQ45.23 ACME205.55 IBM612.78 AAPL

Let's take a look at the effect if zip () is not used?

Prices = {'acme ': 45.23, 'aap': 612.78, 'ibm': 205.55, 'hpq': 37.20, 'fb ': 10.75} print (min (prices )) # Only the print (min (prices. values () # Only search for values. values cannot be mapped to keys. // Add prices = {'aaa': 34, 'bbb ': 34} print (min (zip (prices. values (), prices. keys () # Compare the value size when the value is equal after the reversal

Search for similarities between the two dictionaries

a = { 'x' : 1, 'y' : 2, 'z' : 3}b = { 'w' : 10, 'x' : 11, 'y' : 2}print('Common keys:', a.keys() & b.keys())print('Keys in a not in b:', a.keys() - b.keys())print('(key,value) pairs in common:', a.items() & b.items()

Output:

Common keys: {'y', 'x'}Keys in a not in b: {'z'}(key,value) pairs in common: {('y', 2)}